U.S. Pat. No. 6,171,449 teaches methods of recovering at least a portion of the heat contained in an EB/SM splitter overhead stream via use of a cascade reboiler scheme in which the separation of ethylbenzene and styrene is carried out in two parallel distillation columns operating at different pressures, with the overhead of the high pressure column providing the heat required to reboil the low pressure column.
In contrast, U.S. Pat. No. 4,628,136 teaches a method of recovering the heat contained in the overhead of the EB/SM splitter by using this stream to boil an azeotropic mixture of ethylbenzene and water, which, once vaporized, is subsequently transferred to the reaction system where dehydrogenation of ethylbenzene to styrene takes place. The method described in the U.S. Pat. No. 4,628,136 patent, however, requires that the EB/SM splitter operate at a pressure that is sufficiently high as to enable the transfer of the azeotropic mixture of ethylbenzene and water vapor into the reactor system without the use of a compressor. This patent also specifies that the temperature difference between the condensing EB/SM splitter overhead and the boiling azeotropic mixture of ethylbenzene and water should be in the range of between and 2 and 10° C. Given this temperature constraint, one can derive a relationship between the pressure at which the azeotropic vaporization is taking place and the required overhead pressure of the EB/SM splitter. This relationship is presented graphically in FIG. 4.
As can be seen in the graph, the method taught by U.S. Pat. No. 4,628,136 requires that the EB/SM splitter operate at an overhead pressure of at least 200 mmHg in order for the azeotropic mixture to be transferred into the reactor system without the use of the compressor. This is because the practical lower limit for the pressure at the inlet of the reactor system is of the order of 400 mmHg, and may range up to about 1100 mmHg, which must be increased by another 100 to 200 mmHg in order to pass the azeotropic mixture of ethylbenzene and water vapor through the heat exchange system (e.g., reactor feed-effluent exchanger or a fired heater) which is needed to bring it to the required reaction temperature and to pass this stream into and through the reactor system. As a consequence of this limitation, the method taught by U.S. Pat. No. 4,628,136 results in required operating temperatures for the EB/SM splitter which are significantly higher than in a conventional process where no effort is made to recover heat from the overhead. Operation at such higher temperature and pressure, however, is more costly both in operational and capital costs.
The necessary increase in operating temperature and pressure which is required to practice the method of the U.S. Pat. No. 4,628,136 patent also leads to an increase in the rate of styrene polymerization which is a direct yield loss. For uninhibited styrene monomer, the polymerization rates approximately double for every 7 to 8° C. increase in temperature. In commercial practice, the method taught by U.S. Pat. No. 4,628,136 results in operating the EB/SM splitter at temperatures on the order of 20° C. to 30° C. higher than conventional technology. The net result is either the need for increased dosage rates of costly polymerization inhibitors or accepting an increased formation of undesired styrene polymer (yield loss), or both, resulting in a substantial negative impact on the overall process economics. Furthermore, the close-coupling of the EB/SM splitter and the dehydrogenation reactor system operations required to practice the method of the U.S. Pat. No. 4,628,136 patent means that an increase in pressure drop anywhere in the reaction system (as for example that which may be caused by fouling of heat exchange surfaces or by catalyst attrition leading to higher pressure drop in the catalyst beds) will require that the EB/SM splitter be operated under even higher pressure and temperature conditions than usual, resulting in still further increases in polymerization inhibitor consumption, styrene polymer byproduct, or both.
These and other deficiencies in or limitations of the prior art are overcome in whole or in part by the improved method and related apparatus of the present invention.